hook_node_view

The module may add elements to $node->content prior to rendering. This hook
will be called after hook_view(). The structure of $node->content is a
renderable array as expected by drupal_render().

When $view_mode is 'rss', modules can also add extra RSS elements and
namespaces to $node->rss_elements and $node->rss_namespaces respectively for
the RSS item generated for this node.
For details on how this is used, see node_feed().